roman.py

def roman_numeral_to_int(string):
    symbols = {
        'I': 1,
        'V': 5,
        'X': 10,
        'L': 50,
        'C': 100,
        'D': 500,
        'M': 1000
    }
    repetitions = {}
    result = 0
    skip = 0
    for i, c in enumerate(string):
        if i == skip and i != 0:
            continue
        if c not in symbols:
            return None
        if c in repetitions.items():
            repetitions[c] += 1
        else:
            repetitions = {c: 1}
        for r, v in repetitions.items():
            if (r in ['L', 'D', 'V'] and v > 1) or (r in ['I', 'X', 'C'] and v > 3):
                return None
        if c == 'I':
            # last character in the string
            if i == len(string) - 1:
                result += 1
            elif string[i+1] == 'V':
                result += 4
                skip = i + 1
            elif string[i+1] == 'X':
                result += 9
                skip = i + 1
        elif c == 'X':
            # last character in the string
            if i == len(string) - 1:
                result += 10
            elif string[i+1] == 'L':
                result += 40
                skip = i + 1
            elif string[i+1] == 'C':
                result += 90
                skip = i + 1
        elif c == 'C':
            # last character in the string
            if i == len(string) - 1:
                result += 100
            elif string[i+1] == 'D':
                result += 400
                skip = i + 1
            elif string[i+1] == 'M':
                result += 900
                skip = i + 1
        else:
            skip = 0
            result += symbols[c]
    return result